Switching networks operating with a constant switching frequency and using pulse width modulation are per se known. Such known systems are used for supplying electronic systems with direct current or direct voltage. These prior art switching networks are operated at the frequency of their own internal oscillator.
The aforementioned systems include inter alia digitally operating audio systems, in which the information signal is sampled at a frequency fs and in which the resulting pulse-like signals are then processed. Since the oscillator frequency fo in the supplying network is different from the sampling frequency fs, it is not possible to avoid undesired signals, such as e.g. interference in such systems which have a strong spectral component at the sampling frequency fs and a strong component at the oscillator frequency fo. Such signals can occur as digital signals or as signals sampled with the sampling frequency fs.
More particularly, such interference often occurs in analog - digital converters. The new signals resulting from the interference can be in the audible range of the human ear and the frequency of such interference signals within the audible frequency range can change. More particularly, this occurs if the oscillator frequency fo is not absolutely constant in the switching network. For example, such frequency changes occur in the case of varying loads on the oscillator in the power supply.